selinux: Add support for avc_acquire_netlink_fd()

Requires libselinux 2.0.79 or newer.  Without this, libselinux will
check for policy updates on the netlink socket on basically every policy
lookup.  Statistically speaking, they never happen, and the check
translates to at least one more syscall on basically every operation.

Instead, take control of the fd from the library, and check it in
WakeupHandler if it polls readable.

Xext/xselinux.c

@@ -1886,6 +1886,22 @@ SProcSELinuxDispatch(ClientPtr client)
     }
 }
 
+#ifdef HAVE_AVC_NETLINK_ACQUIRE_FD
+static int netlink_fd;
+
+static void
+SELinuxBlockHandler(void *data, struct timeval **tv, void *read_mask)
+{
+}
+
+static void
+SELinuxWakeupHandler(void *data, int err, void *read_mask)
+{
+    if (FD_ISSET(netlink_fd, (fd_set *)read_mask))
+        avc_netlink_check_nb();
+}
+#endif
+
 
 /*
  * Extension Setup / Teardown
@@ -1916,6 +1932,12 @@ SELinuxResetProc(ExtensionEntry *extEntry)
     label_hnd = NULL;
 
     audit_close(audit_fd);
+#ifdef HAVE_AVC_NETLINK_ACQUIRE_FD
+    avc_netlink_release_fd();
+    RemoveBlockAndWakeupHandlers(SELinuxBlockHandler, SELinuxWakeupHandler,
+                                 NULL);
+    RemoveGeneralSocket(netlink_fd);
+#endif
 
     avc_destroy();
     avc_active = 0;
@@ -2012,6 +2034,13 @@ SELinuxExtensionInit(INITARGS)
     if (atom_client_ctx == BAD_RESOURCE)
 	FatalError("SELinux: Failed to create atom\n");
 
+#ifdef HAVE_AVC_NETLINK_ACQUIRE_FD
+    netlink_fd = avc_netlink_acquire_fd();
+    AddGeneralSocket(netlink_fd);
+    RegisterBlockAndWakeupHandlers(SELinuxBlockHandler, SELinuxWakeupHandler,
+                                   NULL);
+#endif
+
     /* Register callbacks */
     ret &= dixRegisterPrivateInitFunc(subjectKey, SELinuxSubjectInit, NULL);
     ret &= dixRegisterPrivateDeleteFunc(subjectKey, SELinuxSubjectFree, NULL);

configure.ac

@@ -940,6 +940,10 @@ if test "x$XSELINUX" = xyes; then
 	AC_CHECK_LIB(selinux, avc_init, [], AC_MSG_ERROR([SELinux library not found])) 
 	AC_CHECK_HEADERS([libaudit.h], [], AC_MSG_ERROR([SELinux extension requires audit system headers]))
 	AC_CHECK_LIB(audit, audit_open, [], AC_MSG_ERROR([SELinux extension requires audit system library]))
+        AC_CHECK_DECL(avc_netlink_acquire_fd,
+                      [AC_DEFINE(HAVE_AVC_NETLINK_ACQUIRE_FD, 1, "Have avc_netlink_acquire_fd")],
+                      [],
+                      [#include <selinux/avc.h>])
 	AC_DEFINE(XSELINUX, 1, [Build SELinux extension])
 	SELINUX_LIB="-lselinux -laudit"
 fi

include/dix-config.h.in

@@ -425,4 +425,6 @@
 #include "dix-config-apple-verbatim.h"
 #endif
 
+#undef HAVE_AVC_NETLINK_ACQUIRE_FD
+
 #endif /* _DIX_CONFIG_H_ */